1. Field of the Invention
The present invention relates, in general, to a method and apparatus for controlling switched reluctance motors (SRMs) and, more specifically, is directed to a sensorless position and velocity control for SRMs using rotor position estimation based on the measurement of flux and inductance of the phases of the SRM.
2. Description of Related Art
A SRM is a brushless, salient pole electrical machine with multiple poles on both the rotor and stator. The stator has phase windings, unlike the rotor which is unexcited and has no windings or permanent magnets mounted thereon. Rather, the rotor of a SRM is formed of a magnetically permeable material, typically iron, which attracts the magnetic flux produced by the windings on the stator poles when current is flowing through them. The magnetic attraction causes the rotor to rotate when excitation to the stator phase windings is switched on and off in a sequential fashion in correspondence to rotor position.
For a SRM, the direction of the torque of a phase depends on rotor position and phase current magnitude, but not on the polarity of the current. Hence, not every phase of an SRM can produce torque in a desired direction at any given rotor position. When commutating a SRM, it is customary to associate each phase with either a productive phase group (phases which can presently generate torque of a desired direction) or an unproductive phase group (phases which presently can only generate torque of the opposing direction). Naturally, the membership in these phase groups is rotor position dependent and constantly changing. Optimal commutation of a SRM for maximum torque can be produced when every member of the productive phase group is excited and every member of the unproductive phase group remains unexcited.
For example, in a 3-phase SRM, either one or two phases (depending on the rotor position) can produce torque of a desired direction at one time. Therefore, an optimally commutated 3-phase SRM should normally be operated such that, at all times, at least one productive phase is excited and at least one unproductive phase remains unexcited. On the other hand, it should be noted that the optimal commutation of a 3-phase SRM will never lead to the occurrence where there are two or more simultaneously excited phases at all times, or two or more simultaneously unexcited phases at all times.
To provide this optimal commutation and control for a SRM, accurate and timely rotor position information must be obtained. Traditionally, the rotor position information required for stator phase current switching has been provided by such devices as Hall-effect sensors, optical switches, optical encoders, or resolvers. More recently, efforts have been made to dispense with the use of such mechanical position sensors in favor of rotor position estimators based on parameters determinable from the motor terminals. Two parameters which could be usefull in this endeavor are phase flux and phase inductance. They are known to be nonlinear functions of rotor position for a SRM but can be relatively easily calculated from motor phase currents and voltages. However, the rotor position information obtained is a double-valued function (within each electrical cycle) for either phase flux or phase inductance, for any fixed value of phase current. Thus, given a measurement of phase current and a calculation of either phase flux or inductance alone, only a pair of possible rotor positions can be determined.
For example, U.S. Pat. No. 4,772,839 issued to MacMinn (MacMinn I) discloses a rotor position estimator for SRMs which simultaneously measures changes in current in two unexcited phases, processes the measurements to provide a pair of possible rotor positions for each phase, and combines the positions in a fashion which attempts to yield a unique estimate of instantaneous rotor position. But, if the two phases of the SRM do not remain unenergized throughout the sampling period, or if any phase of the motor experiences a change of state during the sampling period, the MacMinn I estimator only provides an extrapolated rotor position in place of the estimated instantaneous rotor position. Moreover, the estimator disclosed in MacMinn I estimates rotor position based on the assumption that the IR drop (voltage drop due to electrical resistance of a phase) and the back EMF (the electromotive force caused by self induction of a phase) terms of the phase voltage equation are so small as to be negligible when estimating phase inductance, whereas such assumptions are not always valid. Specifically, back EMF may be a substantial portion of the phase voltage equation at high rotor velocities.
U.S. Pat. No. 4,959,596 issued to MacMinn, et al. (MacMinn II) discloses a SRM drive system in which a phase inductance sensing technique is used to indirectly estimate certain rotor positions. This technique involves applying voltage sensing pulses to an unexcited phase, which results in a change in phase current. The change in phase current is inversely proportional to the instantaneous phase inductance. Commutation time is determined by comparing the change in phase current to a threshold current, thus synchronizing phase excitation to a relative rotor position. Since no explicit determination of rotor position is obtained, the excitation to the phases cannot be shaped with respect to the position, limiting the control performance that can be achieved (e.g., position control is not possible).
U.S. Pat. No. 5,097,190 issued to Lyons, et al., (Lyons) discloses a rotor position estimator for a SRM based on instantaneous phase flux and phase current measurements. Phase current and flux sensing are performed for the phases in a predetermined sequence that depends on the particular quadrant of SRM operation, i.e., forward motoring, reverse motoring, forward generating, or reverse generating. For each phase in the predetermined sequence of sensing, phase flux and phase current measurements are made during operation in a pair of predetermined sensing regions, each defined over a range of the rotor positions. The rotor position estimates are derived from the phase flux and phase current measurements for each respective phase during the respective sensing regions thereof The rotor position estimates for each phase are normalized with respect to a common reference phase, and a rotor position estimate for the SRM is computed according to an equation which accounts for the fact that for any given rotor position determined, the rotor poles of the SRM may be approaching alignment or unalignment.
Therefore, the prior art generally fails to provide a continuously accurate and unambiguous determination of rotor position for all configurations of SRMs at all times, and a need exists for completely eliminating ambiguity in rotor position estimations for a SRM. It is to the fulfillment of this need that the present invention is directed.